1. Field of the Invention
The present invention relates to a DC/DC converter.
2. Description of the Related Art
Various kinds of electronic devices employ a DC/DC converter that converts a DC voltage having a given voltage value into a DC voltage having a different voltage value. FIG. 1 is a circuit diagram showing a step-down (buck) DC/DC converter. A DC/DC converter 100R receives a DC input voltage VIN via an input terminal 102, and generates an output voltage VOUT having a reduced voltage value at an output terminal 104. The DC/DC converter 100R includes an output circuit 110R and a control circuit 200R. The output circuit 110R mainly includes a switching transistor M1, an inductor L1, a rectifier diode D1, and an output capacitor C1. The output capacitor C1 is connected to the output terminal 104. One end of the inductor L1 is connected to a switching (LX) terminal of the control circuit 200R. The other end of the inductor L1 is connected to the output terminal 104. The rectifier diode D1 is arranged such that its anode is grounded and its cathode is connected to the LX terminal.
The switching transistor M1 is built into the control circuit 200R. A VCC terminal of the control circuit 200R is connected to the input terminal 102. The DC input voltage VIN is supplied to the VCC terminal. The switching transistor M1 is configured as an N-channel MOSFET, which is arranged such that its source is connected to the LX terminal and its drain is connected to the VCC terminal.
A detection terminal (VS) receives, as a feedback signal, a signal that indicates the state (current state, voltage state, electric power state, or the like) of the DC/DC converter 100R or otherwise the state of a load (not shown) connected to the output terminal 104. A pulse generator 202 generates a pulse signal S1 having a factor such as a duty ratio, frequency, or otherwise a combination thereof that is changed such that the state of the DC/DC converter 100R or otherwise the state of the load approaches a target state. For example, in a case in which the DC/DC converter 100R is configured as a constant voltage output DC/DC converter, the pulse generator 202 generates the pulse signal S1 such that the output voltage VOUT approaches a target voltage VREF. In a case in which the DC/DC converter 100R is configured as a constant current output DC/DC converter, the pulse generator 202 generates the pulse signal S1 such that a current IOUT that flows through the load approaches a target value IREF.
A driver 204 switches on and off the switching transistor M1 according to the pulse signal S1. In a case in which the switching transistor M1 is configured as an N-channel MOSFET as described above, in order to turn on the switching transistor M1, there is a need to apply a voltage to the gate of the switching transistor M1 that is higher than the voltage between its drain and source (i.e., input voltage VIN). In order to supply such a voltage, a bootstrap circuit 210 is arranged. The bootstrap circuit 210 includes a bootstrap capacitor C2, a rectifier element 212, a transistor 214, and a bootstrap power supply circuit 220. The bootstrap capacitor C2 is arranged in the form of an external component between the LX terminal and a bootstrap (BST) terminal. The bootstrap power supply circuit 220 generates a constant voltage VCCBST. The rectifier element 212 is arranged between the BST terminal and an output of the bootstrap power supply circuit 220. The transistor 214 is arranged between the LX terminal and the ground. The voltage VBST at the BST terminal is supplied to the upper-side power supply terminal of the driver 204.
During a period in which the switching transistor M1 is turned off, the transistor 214 is turned on, which grounds one end (LX-side end) of the bootstrap capacitor C2. In this state, the voltage VCCBST is applied to the other end (BST-side end) of the bootstrap capacitor C2 via the rectifier element 212. Accordingly, the bootstrap capacitor C2 is charged using the voltage across both ends thereof represented by (VCCBST−VF). Here, VF represents the forward voltage of the rectifier element 212. Such an arrangement is designed such that the relation VCCBST−VF>VGS(TH) holds true. Here, VGS(TH) represents a gate-source threshold voltage of the switching transistor M1.
In the turned-on period of the switching transistor M1, with the source voltage of the switching transistor M1 as VLX, the voltage VBST at the BST terminal is represented by VBST=VLX+(VCCBST−VF). The driver 204 uses the voltage VBST as a high-level voltage to be applied to the gate of the switching transistor M1. In this period, the gate-source voltage VGS is represented by VGS=VBST−VLX=(VCCBST−VF). That is to say, the relation VGS>VGS(TH) holds true. Thus, the switching transistor M1 is turned on.
FIG. 2 is a circuit diagram showing a bootstrap power supply circuit 220R investigated by the present inventors. The bootstrap power supply circuit 220R is configured as a linear regulator (LDO: Low Drop Output) including resistors R21 and R22, an error amplifier 222, and a transistor 224. The transistor 224 is configured as a P-channel MOSFET. It is difficult for such an arrangement to provide high responsivity in the feedback operation. This leads to poor stability of the output voltage VCCBST generated by the bootstrap power supply circuit 220R. In order to solve such a problem, a smoothing capacitor C3 is connected to the output terminal of the bootstrap power supply circuit 220R, which improves the stability of the output voltage VCCBST. Such an arrangement requires a VCCBST terminal (pad) as an additional terminal on the control circuit 200R. Furthermore, such an arrangement requires such a capacitor C3 in the form of an external component. This leads to a disadvantage from the viewpoint of the circuit area and cost.